In the extrusion of pipe or tubing, especially relatively large-diameter pipe or tubing, the tubular extrusion emerging from the extrusion die, to which one or more extruders may be connected to supply the plastified synthetic resin material, e.g. polyvinylchloride, is relatively soft and must be cooled to harden the extrusion into a stable shape. Cooling is commonly effected by contacting the extrusion with a cooling fluid, e.g. water.
Until the extrusion cools sufficiently, it is desirable to maintain a certain air pressure within the extrusion immediately downstream of the die so that the tubular structure or pipe will not collapse or deform.
The air pressure within tubular extrusion may be maintained by forcing air into the extrusion at the die.
The pressure can be maintained by a plug which sealingly engages the inner surface of the extrusion at a location relatively remote from the die and which can be attached flexibly to the die, e.g. via a cable or chain.
One such apparatus utilizing a plug to maintain a pressure upstream of the plug in an extrusion apparatus is described in U.S. Pat. No. 3,296,661.
Plugs for such apparatus have been widely used in practice, although a problem is created when there is a break in the flexible element connecting the plug to the die. Under such circumstances the fluid pressure upstream of the plug may drive the plug uncontrolledly and this is disadvantageous.